This invention relates to an austenitic iron-nickel base alloy and more particularly to such an alloy which can be brought to a high strength level while retaining a unique degree of its toughness.
It has long been sought to increase the strength of large nonmagnetic austenitic parts of thick cross section without substantially increasing the cost of the parts made therefrom which already are expensive. Such large nonmagnetic parts, as for example, retaining rings for the copper windings at the ends of the rotor in large turbine driven electric generators may each weigh several thousand pounds and in addition to being nonmagnetic must also be strong enough to resist the stresses resulting from its own mass and the centrifugal force of the rotating windings.
Heretofore, an alloy containing nominally about 0.5% carbon, 18% manganese, 5% chromium and the balance iron (here designated Alloy A for convenience) has been used to make such large nonmagnetic parts but it has left much to be desired. For one thing, Alloy A cannot be strengthened by heat treatment and consequently has required extensive cold working which is expensive because of the special equipment required to carry out such large amounts of cold work in such large, massive parts. In any event, the upper limit of about 160 ksi 0.2% yield strength (Y.S.) as measured by standard room temperature tensile specimens is not adequate for very large electric generators.
Very large electric generators requiring retaining rings stronger than those which can be provided from Alloy A have been in demand for many years as is evident from the article by K. E. Fritz and D. R. DeForest, "High-Strength Turbo-Generator Retaining Ring Forgings of an Age Hardenable Austenitic Alloy" (Journal of Materials, 1968, pp. 629-645). Fritz and DeForest propose forming such large nonmagnetic parts from A-286 Alloy (A.I.S.I. 660) and show that by combined cold working and aging the 0.2% Y.S. of A-286 Alloy parts can be increased to about 175 ksi. Unfortunately, as Fritz and DeForest point out, the amount of cold working required by parts made from A-286 Alloy to achieve that level of strength is 30 percent. Thus, some improvement in strength can be obtained by using A-286 alloy instead of Alloy A, but the large amount of cold work and the attendant expense remain. The A-286 Alloy as used by Fritz and DeForest had a nominal composition of about 0.05% carbon, 1.25% manganese, 0.5% silicon, 25.5% nickel, 14.75% chromium, 1.3% molybdenum, 0.25% vanadium, 2.3% titanium, 0.25% aluminum, 0.005% boron and the balance iron.